One commonly known class of blood analysis equipment utilizes optical systems to irradiate and analyze individual blood cells. Typically, a small sample of blood is passed through a narrow fluid vortex, approximately one cell wide, a cell at a time at a very high rate. A source of light is focused onto the vortex, such that as cells pass through, they interact with the light beam, absorbing a portion and scattering the rest, depending upon the size, configuration, color, and the like of the cell. In some systems the light source is collimated and non-coherent, whereas in others, lasers are utilized. Further, some systems utilize direct light scattering measurements, whereas others employ the principle of the "dark field" microscope.
In any event, these systems employ an array of photoelectric means which generate a signal based upon the light scattered as the cells pass through the beam of light. Generally, the amplitude of the signal derived from scattered light tends to correlate reasonably well with the cells under analysis. For example, if the system is using the "dark field" concept, signal pulses corresponding to typical red cells are several orders of magnitude (e.g. six to ten times) larger in amplitude than those corresponding to typical platelets. Hence, for some applications, there exists an adequate basis for distinction between red cells and platelets simply based on the amplitude of scattered light.
For many other applications, however, it is beneficial, and sometimes essential, to distinguish red cells from platelets even more closely than upon amplitude discrimination. Thus, it may be useful to distinguish small red cells from large platelets, which would be relatively indistinguishable on a simple amplitude discrimination basis. Likewise, air bubbles, foreign particles, and the like should be recognized as such, and not be identified either as platelets or small red cells.